General-purpose resins such as a polypropylene and polyvinyl chloride using petroleum as raw materials are used in various fields such as miscellaneous goods for daily use, domestic electric products, car parts, building materials and food package materials owing to their characteristics such as good processability and durability. However, when these resin products are scrapped after they perform their duties, the high durability of these resin products is rather the drawback that they are inferior in degradability in the natural world and there is therefore a possibility that they adversely affect a biosystem.
In order to solve such a problem, biodegradable resins such as copolymers of a polylactic acid/lactic acid and other aliphatic hydroxycarboxylic acids and aliphatic polyesters derived from aliphatic polyhydric alcohols and aliphatic polyvalent carboxylic acids have been developed as resins which are thermoplastic resins and biodegradable.
Among these biodegradable resins, polylactic acid resins are expected to be utilized at present because L-lactic acid is produced in a large amount from saccharides extracted from corns and potatoes by the fermentation method so that it is reduced in cost, and the amount of all carbon oxides to be exhausted is very small because its raw materials are natural field crops and also resins obtained from the lactic acid have the characteristics that they have strong stiffness and high transparency. Therefore, these polylactic acid resins are used in the fields of agricultural civil engineering such as flat yarns, nets and raising seeding pots, window envelopes, shopping bags, compost bags, writing materials and miscellaneous goods. However, in the case of these polylactic acid resins, all of these resins are limited in use to hard molding fields because these resins have fragile, hard and less flexible characteristics. When these resins are molded into films and the like, the obtained products have the problems that they have insufficient flexibility and are whitened when they are bent. This is the reason why these resins are only insufficiently spread in soft or semi-hard fields.
Various methods using a plasticizer are proposed as technologies for applying a polylactic acid resin to soft or semi-hard fields. For example, technologies in which a plasticizer such as tributyl acetylcitrate or diglycerin tetraacetate is added are disclosed. When these plasticizers are each added to a polylactic acid to form a film or sheet by extrusion molding or the like, good flexibility is obtained. However, the resin is remarkably changed in flexibility at a temperature close to the glass transition point (sensitivity to temperature) because the resin is in an amorphous state, and also, remarkably changed seasonally in properties because it has unsatisfactory heat resistance at high temperatures, giving rise to the problem that it is difficult to use the resin under a high-temperature circumstance. In order to solve this problem, a method is proposed in which a polylactic acid is crystallized by adding a crystal nucleus agent such as talc (JP-B 3410075) to thereby improve, for example, the heat resistance of the resin. However, this method poses the problem that the rate of crystallization performed by heat treatment after the resin is molded is insufficient, and when a crystal nucleus agent such as talc is added in a large amount, the transparency of a sheet or film after thermal molding is deteriorated and a plasticizer bleeds in the case of allowing a molded article to stand under a high-temperature and high-humidity environment.
In order to solve these problems, JP-A 2006-176747 discloses a biodegradable resin composition containing an aliphatic compound having two or more groups of at least one selected from an ester group, hydroxyl group and amide group as a crystal nucleus agent in a molecule. Also, WO-A 2005/097894 discloses a polylactic acid resin composition containing a metal salt of a phosphorous compound.